1. Field of the Invention
The present invention relates to developer supply rollers and developing units.
2. Description of the Related Art
Conventionally, an image forming apparatus, such as an electrophotographic printer, a copier, or a facsimile machine, includes an exposure unit for exposing a predetermined area of a photosensitive drum uniformly charged to form an electrostatic latent image, a developing unit for developing the electrostatic latent image to form a toner image, and a transfer/fixing unit for transferring the toner image to a recording medium, such as paper, and fixing it thereto.
The developer or toner is depleted or the photosensitive drum or each roller is worn down after repeated printings so that when the developer is depleted or the photosensitive drum or each roller is worn down, the developing unit or EP cartridge that includes a photosensitive drum and each roller and contains a toner is replaced.
FIG. 2 shows a conventional EP cartridge. A photosensitive drum 11 is rotatable and produces an electrostatic latent image on its surface. A developer carrying member or developing roller 12 is disposed so as to contact with the photosensitive drum 11 for supplying the photosensitive drum 11 with a toner 13. A toner layer forming blade 14 is disposed in contact with the developing roller 12 for forming a thin layer of toner 13 on the developing roller 12 to control the amount of toner 13 supplied to the photosensitive drum 11 by the developing roller 12.
A developer supply roller or toner supply roller 15 supplies the developing roller 12 with toner 13 and scraps the toner 13 that is not used for developing a toner image and remains on the developing roller 12. A cleaning blade 16 is disposed in contact with the photosensitive drum 11 to remove the toner 13 remaining on the photosensitive drum 11 after the toner image is transferred to a recording medium. The toner 13 is held by each of the photosensitive drum 11, the developing roller 12, and the toner supply roller 15 by a mirror image force.
A charging roller 17 is disposed in contact with the photosensitive drum 11 for uniformly charging the surface of the photosensitive drum 11 to establish a predetermined surface potential on the photosensitive drum 11. A light source 18 forms a latent image on the photosensitive drum 11.
The photosensitive drum 11, the developing roller 12, the toner layer forming blade 14, the toner supply roller 15, the cleaning blade 16, and the charging roller 17 constitute an EP cartridge in which the toner 13 is contained.
The toner supply roller 15, which not only supplies the toner 13 but also scraps the remaining toner 13 from the developing roller 12, has a foam rubber that has a large number of cellular holes in the surface.
FIG. 3 shows the surface of a conventional toner supply roller. A plurality of cellular holes 15c are formed in the surface of the toner supply roller 15 and a cellular wall 15d is formed between the respective cellular holes 15c. 
When the toner supply roller 15 is rotated in contact with the developing roller 12 (FIG. 2), the toner 13 contained in the cellular holes 15c is supplied to the developing roller 12. The toner 13 remained on the developing roller 12 is scraped into the cellular holes 15c by the cellular wall 15d. 
By increasing the number of cellular holes 15c in a unit area of the toner supply roller 15 or providing a spiral projection on the toner supply roller 15, it is possible to increase the power for scraping the remaining toner 13 from the developing roller 12. See JP 2000-56556 for example.
However, it is necessary to reduce the diameter of the cellular holes 15c in order to increase the number of cellular holes 15c. In addition, if the diameter of cellular holes 15c is too small, the amount of toner 13 in the cellular hole 15c becomes too small to supply the developing roller 12 with the satisfactory amount of toner 13.
The spiral projection formed on the toner supply roller 15 needs a special forming mold or the application of a heat ray to the toner supply roller 15, making the operation complicated and raising the manufacturing cost of the EP cartridge.